Oxidizing Capacity of Iron Electrocoagulation Systems for Refractory Organic Contaminant Transformation

Abstract

Iron electrocoagulation (Fe EC) is normally considered as a separation process. Here, we found that Fe­(II)–O2 interactions in Fe EC systems could produce reactive oxidants, mainly hydroxyl radicals (•OH), for refractory organic contaminant transformation. Production of reactive oxidants, probed by benzoate conversion to p-hydroxybenzoic acid (p-HBA), depended on dissolved oxygen (DO) concentration and Fe­(II) speciation. Measurable levels of DO were required for significant p-HBA production. Fe precipitates evolved from lepidocrocite to magnetite when DO decreased to below the detection limit. Both experiments and kinetic modeling suggest that the main Fe­(II) species contributing to reactive oxidants (mainly •OH) production changed from aqueous Fe­(II) initially to lepidocrocite-sorbed Fe­(II) with progressive precipitates formation. When DO was not measurable at high currents (≥50 mA or 100 mA/L), reactive oxidant production was ineffective because of pH rise and Fe­(II) preservation in magnetite, but it could be enhanced drastically by aeration. The reactive oxidants produced at 30 mA (or 60 mA/L) could degrade about 47% of 10 μM aniline and 34% of sulfanilamide within 6 h of Fe EC treatment. Our findings highlight the importance of reactive oxidants for refractory organic contaminants oxidation in Fe EC systems.

Electrochemical technologies for per‐ and polyfluoroalkyl substances mitigation in drinking water and water treatment residuals

“…For example, the direct oxidation of organic pollutants at the anode surface has been observed previously, indicated by an irreversible oxidation peak below the oxygen evolution potential during cyclic voltammetry (Linares-Hern andez et al, 2009). Qian et al (2019) investigated the oxidizing capacity of iron-electrocoagulation using a combination of kinetic modeling and quantifying the conversion of benzoate to p-hydroxybenzoic acid (an oxidation byproduct for mechanism analysis). The authors concluded that electrocoagulation may yield reactive oxidants (such as hydroxyl radicals) via Fenton-like mechanisms (Qian et al, 2019).…”

“…Electrocoagulation may offer both nondestructive and destructive PFAS treatment by electrolysis of sacrificial anode materials such as iron, aluminum, or zinc (Figure 2). Electrocoagulation has classically been studied as a phase separation/nondestructive removal technology although several studies indicated that it may also serve as a destructive/oxidative removal technology for trace organic compounds that are recalcitrant to sorption such as estrogenic compounds, acetaminophen, atenolol, and bronopolol (Bocos et al, 2016;Govindan et al, 2020;Kim et al, 2020;Maher et al, 2019;Qian et al, 2019).…”

“…Electrocoagulation may offer both nondestructive and destructive PFAS treatment by electrolysis of sacrificial anode materials such as iron, aluminum, or zinc (Figure 2). Electrocoagulation has classically been studied as a phase separation/nondestructive removal technology although several studies indicated that it may also serve as a destructive/oxidative removal technology for trace organic compounds that are recalcitrant to sorption such as estrogenic compounds, acetaminophen, atenolol, and bronopolol (Bocos et al, 2016; Govindan et al, 2020; Kim et al, 2020; Maher et al, 2019; Qian et al, 2019). Electrocoagulation has previously been applied for drinking water treatment studies focused on heavy metals, estrogens, natural organic matter (NOM), and disinfection byproducts; these studies are beneficial for demonstrating performance based on drinking water metrics (Dubrawski & Mohseni, 2013a; Heffron et al, 2016; Maher et al, 2018; McBeath, Mohseni, & Wilkinson, 2020; Mohora et al, 2012; Ryan et al, 2020; Vik et al, 1984).…”

“…Qian et al (2019) investigated the oxidizing capacity of iron‐electrocoagulation using a combination of kinetic modeling and quantifying the conversion of benzoate to p ‐hydroxybenzoic acid (an oxidation byproduct for mechanism analysis). The authors concluded that electrocoagulation may yield reactive oxidants (such as hydroxyl radicals) via Fenton‐like mechanisms (Qian et al, 2019).…”

“…Qian et al (2019) investigated the oxidizing capacity of iron‐electrocoagulation using a combination of kinetic modeling and quantifying the conversion of benzoate to p ‐hydroxybenzoic acid (an oxidation byproduct for mechanism analysis). The authors concluded that electrocoagulation may yield reactive oxidants (such as hydroxyl radicals) via Fenton‐like mechanisms (Qian et al, 2019). A combination of electrochemical experiments to verify working electrode potentials and oxidation byproducts parameterization is needed to substantiate the production of oxidants during iron‐electrocoagulation.…”